Urotropin azelate: a rather unwilling co-crystal

Area of Science:

• Crystallography and Materials Science
• Solid-state chemistry
• Phase transitions

Background:

• Urotropin (U) and azelaic acid (AA) form 1:1 co-crystals (UA) with complex diffraction patterns.
• UA contains defects like solvent inclusions, parasite phases, and high vacancy/dislocation densities, influencing its properties.

Purpose of the Study:

• To investigate the complex phase transitions in Urotropin-Azelaic acid (UA) co-crystals.
• To understand the defect-driven transitions and model the order-disorder (OD) structure.

Main Methods:

• X-ray diffraction analysis to characterize co-crystals and their complex diffraction patterns.
• Differential scanning calorimetry (DSC) to detect phase transitions.
• Three-dimensional Monte Carlo modeling to simulate the OD structure and interactions.

Main Results:

• UA exhibits at least seven exotic phase transitions, including incommensurate phases and a peritectoid reaction.
• The system behaves as an OD structure with transitions from a disordered state at 338 K to ordered orthorhombic, monoclinic, and ground states at lower temperatures.
• A Monte Carlo model successfully reproduces observed diffraction patterns, accounting for interactions and defects.

Conclusions:

• The complex phase behavior of UA co-crystals is driven by defects and molecular mobility.
• The observed transitions can be modeled by interactions within an OD structure, validated by Monte Carlo simulations.